Pluto
is the most mysterious planet in our solar system. It was not discovered
by a professional astronomer, but it was found by a farm boy in 1930.

Mathematicians predicted the
position of Neptune, the 8th planet, because
of the bumpy orbit of Uranus. But within half
a century, astronomers were up against the same unnatural behaviour again.
Not only was Uranus still wandering in a bumpy orbit, so was Neptune.
It was time to send in the mathematicians. Percival Lowell was an eccentric
Boston citizen who was mad about astronomy. He looked for the 9th planet,
both with pencil and paper, and his new telescopes.

His first search for this mysterious
9th planet ran from 1905 to 1909 - and it was unsuccessful. His next survey
of the sky began in March, 1911. As he continued to fine tune his predictions,
he kept on sending his results to his observatory in Arizona. Pluto was
actually photographed in 1915 on two occasions - on March 19th and on
April 7th. But whoever was looking at the photos must have been half-asleep
and didn't notice the 9th planet. They missed out on their claim to fame.

In 1916, Percival Lowell died.
By 1929, the observatory had installed a new and larger telescope, but
it had also ran out of money. Luckily, Clyde Tombaugh, a 22 year-old Kansas
farm boy, applied to work at Lowell's observatory in Arizona. In his application
he sent them drawings of various objects that he had seen in the heavens
with his own homemade telescope. He was a stroke of luck. He had a good
sharp eye, he could keep the fires burning and clean out the offices,
and best of all, because he was not a professional astronomer they wouldn't
have to pay him much at all. The observatory hired him immediately. He
settled into the new routine very rapidly. At night he would photograph
the skies, and by day he would check the photos for movement. Each plate
had half a million stars. On April 11th 1929, Pluto appeared on one of
his plates, but he did not notice it.

But fate gave him a second
chance and on February 18th, 1930, he found a tiny dot that moved the
right amount. He had done that had only been done twice before in recorded
human history - he had discovered a new planet. He told his boss of this,
and that night went to see Gary Cooper starring in a new movie called
"The Virginian". The Astronomical Union called this planet Pluto after
the Dark God of the Underworld, the world of eternal darkness. It's a
good name, because firstly it's very dark in the outer solar system, and
secondly the first two letters of Pluto were Percival Lowell's initials,
P L.

Pluto was only 5o away from
the predicted position, but there was small problem. Pluto was much too
small and light to have caused the bumps in the orbits of those giant
planets Uranus and Neptune - but by an amazing coincidence, Clyde found
it in the predicted place.

The next major discovery about
Pluto was made in 1979, when James Christy of the United States Naval
Observatory suddenly noticed that photographs of Pluto had a bulge on
one side. And when he had a look at photographs taken a few days before,
he noticed the bulge was on the other side. Pluto had a moon! It spun
around Pluto every 6.4 days.

He named this moon after Charon
(pron. KARON), who in Greek Mythology was the ferryman who rowed the souls
across the river Styx from the world of the living to Pluto's world of
the dead, Hades. But because his wife, Charlene was known as Char, he
proposed that in future everyone should pronounce "KARON" as "SHAR-ON".
His delighted wife said "Some husbands promise their wives the moon, but
mine got it for me". Pluto is about 2,300 km across, and it's nearly twice
as dense as water. About 75% of Pluto is rock, about 20% is water-ice,
and about 5% is methane-ice. Pluto has a rocky core, with a layer of water-ice
and with methane on the top.

Charon is about half the diameter
of Pluto, about 1,300 km across. It is greyish in colour and seems to
have absolutely no methane on it, only water-ice.

Because Charon is about half
the diameter of Pluto, we shouldn't call them a planet and moon, but a
double-planet system. Pluto and Charon are about 20,000 km away from each
other. The next major discovery for Pluto happened on June, 9th, 1988.
For a period of only 18 seconds, just after midnight, Pluto passed in
front of a very faint star. This eclipse was visible from the South Pacific
region, including Australia and New Zealand. Temporary astronomical observatories
were set up in Australia and New Zealand. NASA called its flying observatory,
a converted Lockheed transport plane, into action as well. The astronomers
noticed that as Pluto swung in front of this distant star, the light didn't
switch off suddenly, but dimmed over a few seconds. And then the light
brightened again slowly on the other side. They reckon that this means
that Pluto has an atmosphere. The atmosphere is in two layers - it has
a 50 kilometre thick layer close to the surface, and a thin layer 300
kilometres thick on top of that. But the "air" is few million times thinner
than the earth's atmosphere. The temperature of the atmosphere is about
68 degrees above absolute zero, -250oC.

Pluto takes around 248 years
to travel around the sun, but for about 20 years of each orbit, it cuts
inside the orbit of Neptune. It last did this in 1979, and had its closest
approach to the sun in 1989. It is now heading out and will cross the
orbit of Neptune in 1999, and will once again become the most distant
planet in the Solar System.

But even though Pluto cuts
inside the orbit of Neptune, it will never collide with Neptune. Pluto
has very tilted orbit, so it always misses Neptune. And the two planets
are locked together into a stable resonance orbit. While Pluto takes 248
years to go around the Sun, Neptune takes exactly two thirds as long.
So everytime Neptune does three revolutions of the sun, Pluto has done
two orbits, and they're back to their original starting positions, relative
to each other.

Because Pluto has such an egg-shaped
orbit, it receives much more sunlight when it is closest to the Sun.
In fact, Pluto is the only planet with an atmosphere that evaporates up
from frozen solids, and then condenses back into a frozen solid during
each orbit.